Abstractions of Web Services

Abstract

A particular challenge which is critically important to the development and reusability of Web Service (WS) systems is to have a precise understanding of the functionality of the service under consideration. Currently, this information is not captured by the associated WS technologies. For instance, the WSDL description at best captures type information associated with each operation provided by the WS but it does not specify its behavior. Without a precise understanding of the functionality of a WS, we cannot have high confidence in reusability. In this paper, we present an abstract modelling framework specifically tailored for WS. The framework takes advantage of the characteristic behavior of a WS as a system that repeatedly takes a request and produces an appropriate response in order to simplify the abstraction. The approach uses two complementary models: one is state based, described in Z notation, and the other is event-based, expressed in the process algebra of Hoares communicating sequential processes (CSP). The former will be used to capture the state of the WS and to model back- end operations on it whereas the latter will be used to model behavior, and in particular, front-end interactions and communications. The abstraction makes a careful distinction between operations offered by a WS, therefore appear in its WSDL description and interface, and those which are only used through hidden interactions with other WSs and, therefore do not appear in its WSDL description. The modelling is illustrated through the example of a currency conversion system. The CSP descriptions are developed into WSs implementations using current technologies such as SOAP under Java 2 Enterprise Edition (J2EE) and Microsoft .NET framework. Having abstract models for WSs can lead to a precise understanding of their functionalities and enable assurance in their reusability.